Remember this now, Mr Banks?

When John Banks uttered the words "I don't recall" his credibility evaporated.

The words are recognised the world over as a fraying safety net for the guilty. Banks insisted, "nothing to fear, nothing to hide".

But he did. And it came tumbling out in a thick file of police witness statements, this week.

The statements show that Banks' memory was working just fine in February. In an act of self-preservation (which perversely prompted vengeance from a snubbed Kim Dotcom), he told the jailed millionaire's lawyer that it wouldn't look good to publicly help him out because of "support" given to Banks' mayoralty bid.

Weeks later, his memory failed him. "I don't know who gave me money, I can't remember now,” he said of campaign donations.

He couldn't recall a helicopter ride or lunches chez Dotcom. Mona Dotcom was "the most beautiful woman" he'd met - but even that didn't jog his memory.

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In January, after a police raid that made headlines worldwide, Banks told reporters he hardly knew Dotcom. He must have forgotten that a few weeks earlier he'd enjoyed Dotcom's "hospitality" on a Hong Kong holiday.

Banks' synapses also failed to fire over another donation. Despite attending a meeting with Sky City's chief executive Nigel Morrison, and accepting a Sky City-branded envelope, Banks didn't make the connection between that and a $15,000 gift (a donation his campaign later supplied a receipt for).

Now, for a 65-year-old with a memory problem, those local electoral laws could be a bit confusing. Banks this week said he was a victim of poorly drafted legislation.

But the former police minister understood enough to manipulate those laws. He kept financial transactions "at arm's length". His treasurer stressed to police Banks "asked not to know". "He would tell us off if we ever mentioned donations."

Banks clearly understood enough about this "unclear . . . unworkable" law to know how he could solicit donations and not publicly declare them.

Ministerial office requires not just adherence with the law. It demands the highest ethical standards. Nick Smith learned that when he got bumped this year.

Banks, however, clings on to office because Key needs his vote. But there are signs he is tiring of ACT, especially as internal polls put them at a big fat zero. The Government last month shelved the Budget spending cap - a cornerstone of its coalition deal with ACT.

The party has made no progress in rejuvenating its brand since the election. A quick kill and a neat by-election would maintain Key's majority. And any loyalties to Banks can be quickly forgotten.